ACID is thought to have set off the explosion inside a laboratory cupboard at Clinton, which is used by a chemical testing company.

And Workplace Health and Safety Queensland (WHSQ) yesterday launched an investigation into the cause of the explosion - and if safety procedures were followed - in the early hours of Thursday morning at Intertek, a company providing gas quality testing services to the region's major LNG producers.

The explosion occurred in a laboratory described by Gladstone fire station officer Graham Smith as a "room within a room" which is believed to have contained the inital blast.

But firefighters still rushed staff - who had just arrived at work to find the explosion - out of the "the immediate area" to the building's office section "until we investigated".

He said when the "owner" of the building wasn't able to decipher what chemical was behind the explosion, fire fighters worked to "isolate which one had failed".

"To do that, we had two fire fighters in chemical suits take a photograph of the cabinet and after a process of elimination it was determined it was an acid spill," he said.

"It's still unknown what caused the initial explosion.

"Everyone's bemused or confused about why it's occurred."

But WHSQ has sent in its own inspectors.

"These are procedures that need to be followed through. Where it's in a workplace and an incident like that happens - particularly with chemicals involved," a WHSQ spokesman said.

"They'll be having a look at all aspects. I can't comment on what they'll be having a look at. But they'll be looking at why, what, all those sort of things."

In a statement, Intertek said it "had taken all possible precautions to protect staff" and was working closely with WHSQ through its investigation.